How to toggle visibilty of Subsurf modifier using shortcut?
Toggle Simplify on/off

One easy approach is creating a shortcut for Simplify, which also affects subdivision display settings in the 3D View. The following Add-on  creates the shortcut to toggle Simplify on/off automatically. Once the Add-on is enabled, you can press ShiftQ:


Click to enlarge


Download the latest version
Extract the archive
Install blender-toggle-simplify.py via File > User Preferences > Add-ons
Change the key configuration according your personal preference






Toggle Subsurf Visibility on/off

Another approach would be to toggle the visibility  of the objects subsurf modifier(s) on and off. Depending on the current state, the following Add-on allows to press ShiftQ to enable/disable the Eye property of all subsurf modifiers. If no subsurf modifier is present, a new one will be created:


Click to enlarge


Download the latest version
Extract the archive
Install blender-subsurf-visibility-toggle.py via File > User Preferences > Add-ons
Change the key configuration according your personal preference